I've had 4 different experiences with Turbo Buick & transmissions.
Car #1 - '86 T-type
3.42 gears / 275/50's
Bone stock trans - shifts were mushy like a snowmobile
Car#2 - '87 GN
3.42 gears / 28x10.5's
Bone stock 200R4 - broken when I bought car. (no 2nd gear)
Swapped to a TH400 w/transbrake - went easy 10.90's - missed having the overdrive, but the bulletproof reliability was an acceptable trade-off.
Car #3 - '87 GN (see sig)
3.42 gears / 275/60's
Modded 200R4 - fresh built before I bought the car.
So far I'm impressed, but I am concerned about long-term reliability and the TV cable adjustment. I'm considering a reverse manual valve body to eliminate the TV cable. (I can't find the "happy place" for the TV cable... one extreme to the other)
The overdrive is really growing on me since I've put almost 1500 miles on the car since February. If there is a way to make the 200R4 survive into the 9's, I'll keep it in the car. Otherwise, I have no other choice but to replace it with a TH400 and live without the extra gear.
I'd consider a 4L80E... but at the moment they seem to be a bit expensive.
And, in case you were not aware, big gears don't work well in these cars. I think a 3.23 - 3.42 gear is about perfect.